A numerical model of ground-water flow in the Twin Cities metropolitan area was converted from the TrescottLarson computer code to the McDonald-Harbaugh computer code to facilitate current and future use of the model using up-to-date computer software and hardware. Differences exist between the two computer codes in how headdependent source-sink functions, including leaky rivers, springs and seepage faces (drains), and head-dependent flux boundaries, are formulated. Because of differences in the formulation and calculation of the conductance terms, the conductance values for the head-dependent source-sink functions from the Trescott-Larson Twin Cities model were multiplied by the area of the appropriate model cell for conversion to the McDonald-Harbaugh Twin Cities model. Leaky rivers were simulated using the river package in the McDonald-Harbaugh Twin Cities model, springs and seepage faces were simulated using the drain package, and head-dependent flux boundaries were simulated using the general-head boundary package.

Hydraulic heads and flows in the aquifer system calculated by the McDonald-Harbaugh Twin Cities model were compared to those calculated by the Trescott-Larson Twin Cities model to verify that the results calculated by the two models are similar. Mean differences in calculated hydraulic heads for the drift, drift and St. Peter aquifer, and drift and Prairie du Chien-Jordan aquifer model layers ranged from 1.1 to 1.6 feet. Mean differences in calculated hydraulic heads for the drift and Ironton-Galesville aquifer, drift and Eau Claire confining unit, and drift and Mount Simon-Hinckley aquifer model layers were 0.4 feet or less. Large differences in calculated hydraulic heads at a few cells probably were due to differences in the solution methods used in the McDonald-Harbaugh and Trescott-Larson computer codes to calculate hydraulic heads at and near model layer boundaries.

Differences in calculated flow rates for the McDonald-Harbaugh and Trescott-Larson Twin Cities models were less than 0.3 cubic feet per second for recharge and the head-dependent source-sink functions. Differences in calculated flow in and flow out of constant-head cells for each model layer for the two models ranged from about 2 cubic feet per second for the Mount Simon-Hinckley aquifer model layer to about 45 cubic feet per second for the Prairie du ChienJordan aquifer model layer. The differences between the net flow rates at constant-head cells calculated by the two models for each model layer were much smaller, ranging from 0.01 cubic feet per second for the Ironton-Galesville and Mount Simon-Hinckley aquifer model layers to 8.39 cubic feet per second for the St. Peter aquifer model layer. Differences in the calculated ground-water budgets for the two models were 5.3 percent for the total sources and 4.4 percent for the total sinks.